Vapor pressure deficit control for food preservation

ABSTRACT

A method for control of vapor pressure deficit for food preservation in a refrigerator. The method includes measuring the temperature and humidity of the food storage compartment, determining the vapor pressure deficit in the food storage compartment, and adjusting the temperature in the food storage compartment to maintain the vapor pressure deficit within the desired range.

FIELD OF THE INVENTION

The present disclosure relates generally to a method for control ofvapor pressure deficit for food preservation in a refrigerator.

BACKGROUND OF THE INVENTION

Typically, refrigeration is used to maintain a food storage compartmentat a desired temperature to reduce food decay and increase the usefullife of stored food products. However, temperature is not the onlyvariable affecting food preservation. Humidity is also critical toeffective food preservation and storage. For example, fresh fruits andvegetables are living tissues. Although they are no longer attached to aplant, they breathe, and their composition and physiology continue tochange after harvest. The cellular breakdown and death of fresh fruitsand vegetables are inevitable but can be slowed with optimal storageconditions. Water loss results in weight loss, wilting, and shriveling,while free water or condensation facilitates pathogen growth.

In a refrigerator, it is common to control the temperature of acompartment within a tight band. However, controlling humidity is muchmore difficult because a refrigerator is not equipped with an activehumidifier. Accordingly, strict temperature control in a refrigeratorcan lead to an unfavorable vapor pressure deficit condition within thefood storage compartment that leads to accelerated spoilage of thestored food products.

BRIEF DESCRIPTION OF THE INVENTION

The present subject matter provides a method for control of vaporpressure deficit for food preservation in a refrigerator. The methodincludes measuring the temperature and humidity of the food storagecompartment, determining the vapor pressure deficit in the food storagecompartment, and adjusting the temperature in the food storagecompartment to maintain the vapor pressure deficit within the desiredrange. Aspects and advantages of the invention will be set forth in partin the following description, may be apparent from the description, ormay be learned through practice of the invention.

In one example aspect, a method for preserving food inside arefrigerator is provided. The refrigerator including a compartment forstoring food. The method includes measuring the humidity in thecompartment and measuring the temperature in the compartment. The vaporpressure deficit in the compartment is determined. The temperature inthe compartment is adjusted to maintain the vapor pressure deficit inthe compartment within a desired range.

In another example aspect, a method of preserving food inside arefrigerator is provided. The refrigerator includes a plurality ofcompartments for storing food. The method includes measuring thehumidity in each of the plurality of compartments and measuring thetemperature in each of the plurality of compartments. The vapor pressuredeficit is determined in each of the plurality of compartments. Vaporpressure deficit control is selected for at least one of the pluralityof compartments. For each of the plurality of compartments for whichvapor pressure deficit control is selected, the temperature isindividually adjusted in each of the selected compartments to maintainthe individual vapor pressure deficit in each of the selectedcompartments within an individually desired range. For each of theplurality of compartments for which vapor pressure deficit control notselected, the temperature is individually adjusted in each of theunselected compartments within an individually desired range withoutregard to the vapor pressure deficit in any of the unselectedcompartments.

In another example aspect, a refrigerator for storing food is provided.The refrigerator includes at least one food storage compartment; atleast one thermometer; at least one hygrometer; a sealed systemconfigured for cooling air in the at least one food storage compartment;and a controller. The controller is configured to: 1) receive a measuredtemperature from the thermometer and a measured humidity from thehygrometer and to calculate a vapor pressure deficit based on themeasured temperature and measured humidity and 2) activate or deactivatethe sealed system. When the vapor pressure deficit is within a desiredrange, the controller deactivates the sealed system. When the vaporpressure deficit is outside of the desired range, the controlleractivates the sealed system. The controller may be configured to repeatthe determination of the vapor pressure deficit and activate ordeactivate the sealed system periodically to maintain the vapor pressuredeficit within the desired range, as long as the temperature is withinthe limits for safe food storage.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures.

FIG. 1 is a front view of a refrigerator appliance according to anexample embodiment.

FIG. 2 is a schematic representation of an example embodiment of arefrigerator of the present subject matter.

FIG. 3 is a graphical representation of temperature and vapor pressuredeficit under typical tight temperature control of a refrigerator.

FIG. 4 is a graphical representation of temperature and vapor pressuredeficit under tight vapor pressure deficit control of a refrigerator ofan example embodiment of the present subject matter.

DETAILED DESCRIPTION OF THE INVENTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

As used herein, the terms “includes” and “including” are intended to beinclusive in a manner similar to the term “comprising.” Similarly, theterm “or” is generally intended to be inclusive (i.e., “A or B” isintended to mean “A or B or both”). Approximating language, as usedherein throughout the specification and claims, is applied to modify anyquantitative representation that could permissibly vary withoutresulting in a change in the basic function to which it is related.Accordingly, a value modified by a term or terms, such as “about,”“approximately,” and “substantially,” are not to be limited to theprecise value specified. In at least some instances, the approximatinglanguage may correspond to the precision of an instrument for measuringthe value. For example, the approximating language may refer to beingwithin a ten percent (10%) margin.

FIG. 1 is a front view of a refrigerator appliance 100 according to anexemplary embodiment. Refrigerator appliance 100 includes a cabinet orhousing 10 that extends between a top portion 12 and a bottom portion 14along a vertical direction V. Housing 10 defines chilled food storagecompartments for receipt of food items for storage. In particular,housing 10 defines a fresh food storage compartment 20 positioned at oradjacent top portion 12 of housing 10 and a freezer storage compartment22 arranged at or adjacent bottom portion 14 of housing 10. As such,refrigerator appliance 100 is generally referred to as a “bottom mountrefrigerator.” It is recognized, however, that the benefits of thepresent disclosure apply to other types and styles of refrigeratorappliances such as, e.g., a top mount refrigerator appliance, aside-by-side style refrigerator appliance, or a stand-alone ice makerappliance. Consequently, the description set forth herein is forillustrative purposes only and is not intended to be limiting in anyaspect to any particular chilled storage compartment configuration.

Refrigerator doors 30 are rotatably hinged to an edge of housing 10 forselectively accessing fresh food chamber 20. In addition, a freezer door132 is arranged below refrigerator doors 130 for selectively accessingfreezer chamber 22. Freezer door 32 is coupled to a freezer drawer (notshown) slidably mounted within freezer chamber 22. Refrigerator doors 30and freezer door 32 are shown in a closed configuration in FIG. 1 .

Refrigerator appliance 100 also includes a dispensing assembly 40 fordispensing liquid water and/or ice. Dispensing assembly 40 includes adispenser 42 positioned on or mounted to an exterior portion ofrefrigerator appliance 100, e.g., on one of doors 30. Dispenser 42includes a discharging outlet 44 for accessing ice and liquid water. Anactuating mechanism 46, shown as a paddle, is mounted below dischargingoutlet 44 for operating dispenser 42. In alternative exemplaryembodiments, any suitable actuating mechanism may be used to operatedispenser 42. For example, dispenser 42 can include a sensor (such as anultrasonic sensor) or a button rather than the paddle. A user interfacepanel 48 is provided for controlling the mode of operation. For example,user interface panel 48 includes a plurality of user inputs (notlabeled). Discharging outlet 44 and actuating mechanism 46 are anexternal part of dispenser 42 and are mounted in a dispenser recess 50.

FIG. 2 provides a schematic representation of an example embodiment of arefrigerator of the present subject matter. In an example embodiment ofthe present subject matter, the refrigerator 100 includes at least onefood storage compartment 110, an evaporator 120, an evaporator fan 130,a compressor 140, a condenser 150, a condenser fan 160, and an expansiondevice 170. The evaporator 120, compressor 140, condenser 150, andexpansion device 170 may collectively form a sealed system. Theevaporator 120, compressor 140, condenser 150, and expansion device 170are in fluid communication via appropriate pipes/tubes 210 to allow thecycling of the refrigerant (not shown) to produce cooling in the foodstorage compartment 110 as is known in the art. In this exampleembodiment, the refrigerator 100 further includes a thermometer 180, ahygrometer 190, and a controller 200. In the example embodiment, thecontroller 200 is in signal and/or operative communication with thethermometer 180, hygrometer 190, evaporator 120, evaporator fan 130,compressor 140, condenser 150, and condenser fan 160. In operation, thethermometer 180 measures the temperature in the food storage compartment110 and the hygrometer 190 measures the humidity in the food storagecompartment 110. The temperature and humidity values are communicated tothe controller 200.

The refrigerator 100 may also include data storage device (not shown)and a food detection device (not shown) in signal and/or operativecommunication with the controller. In embodiments, the food detectiondevice is any sensor capable of identifying the food(s) contained in thefood storage compartment without the active input of the user. Examplesof food detection devices include cameras, RFID readers, bar codescanners, and the like.

The controller 200 may be adapted to allow different modes of operationfor the refrigerator 100. In an exemplary embodiment, the refrigerator100 may be operated in one of four (4) modes—standard mode, manual VPDcontrol mode, semiautomatic VPD control mode, and automatic VPD controlmode. As used herein, “standard mode” may correspond to a mode whereinthe refrigerator operates to tightly control temperature without regardto the VPD, e.g., as is typical in known refrigerators. As used herein,“manual mode” may correspond to a mode wherein the refrigerator operatesto control VPD, in the manner described herein, and the desired VPD anddesired temperature range for the compartment are selected by the user.As used herein, “semiautomatic mode” may correspond to a mode whereinthe refrigerator operates to control VPD, as described herein, and theuser selects a food or food type and the VPD and temperature range forthe compartment are selected by the controlled based on data stored inthe controller (or a data storage device). As used herein, “automaticmode” may correspond to a mode wherein the refrigerator operates tocontrol VPD, as described herein, and refrigerator identifies thefood(s) stored in the compartment with a food detection device and theVPD and temperature range for the compartment are selected by thecontroller based on the identified food(s) and data on the appropriateVPD for various food(s) stored in the controller (or the data storagedevice) without active input by the user.

In example embodiments, the user may select the desired mode ofoperation the refrigerator by any convenient mechanism. For example, therefrigerator may include local controls (e.g., buttons or a control pad)to select the desired mode and other required inputs (e.g., desired VPDand/or temperature range or food type). Alternatively, the refrigeratormay be connected to a smart device, such as a smart phone, tablet,computer, e.g., via Wi-Fi or Bluetooth, and the mode of operation of therefrigerator and other inputs may be selected by the user using thesmart device.

When required for any of the VPD control modes, the controller 200determines the vapor pressure deficit (VPD) as discussed in greaterdetail below. The controller 200 compares the VPD to a desired VPDchosen by the user using an input 220 or determined by the controller200 (as discussed below). The controller 200 then determines thetemperature required to achieve and maintain the VPD in the desired VPDrange. The controller 200 then engages or activates evaporator 120,evaporator fan 130, compressor 140, condenser 150, and condenser fan 160to cool the food storage compartment 110 or disengages or deactivatesthe evaporator 120, evaporator fan 130, compressor 140, condenser 150,and condenser fan 160 to allow the temperature in the food storagecompartment 110 to rise, as appropriate to achieve and maintain the VPDin the desired VPD range. In this example embodiment, the above stepsare repeated periodically to maintain close VPD control.

In an example embodiment of the present subject matter, the controller200 determines the VPD using the Tetens equation to calculate thesaturation vapor pressure (P_(s)). (Teten, O., Uber einigemeteorologische, Begriffe Z. Geophys, 6: 297-309 (1930), which isincorporated by reference in its entirety). This equation states:

P _(s)(T)=0.61078 exp^((17.269 T/(T+273.3)))

where:

-   -   P_(s)(T) is the saturation vapor pressure (P_(s)) in kPa at the        temperature (T); and    -   T is the temperature in degrees Celsius reported by the        thermometer 180.

Using the calculated saturation vapor pressure (P_(s)) at the knowntemperature (T), the actual vapor pressure (P_(VAP)) may be calculated:

P _(VAP) =P _(s)(T)*H/100

where:

-   -   P_(VAP) is the actual vapor pressure in kPa and    -   H is the humidity as measured by the hygrometer 190.

Finally, the VPD can be determined by subtracting the actual vaporpressure from the saturation vapor pressure

VPD=P _(s)(T)−P _(VAP)

where:

-   -   VPD is the vapor pressure deficit in kPa,    -   P_(s)(T) is the saturation vapor pressure (P_(s)) in kPa at the        temperature (T), and    -   P_(VAP) is the actual vapor pressure in kPa.

The loss of water from a food product and/or presence of free watercondensation is related to the VPD, i.e., the difference between theactual humidity of the surrounding air and the maximum amount of waterthat the air can hold. While the above definition for VPD is commonlyused, the primary driving force for moisture loss from food is the watervapor pressure difference that exists between the food surface and itssurroundings. Thus, the most relevant vapor pressure difference at thefood surface is:

VPD _(surf) =P _(s)(T _(surf))−P _(s)(T)*H/100

-   -   where T_(surf) is the surface temperature of the food,    -   VPD_(surf)=VPD at the surface of the food, and    -   T is the temperature of the surrounding air.

In most scenarios, a reasonable approximation can be made that thesurface of the stored food item (T_(surf)) is at the same temperature asthe storage compartment as reported by the thermometer (i.e.T_(surf)=T). In such situations, VPD_(surf) equals VPD.

However, if this is not true, an offset value may be added to thethermometer measurement based on the type of food stored to determinethe approximate surface temperature of the food such thatT_(surf)=T+Offset. As noted above, fresh fruits and vegetables areliving tissues. And, although the fruits and vegetables are no longerattached to a plant, the fruits and vegetables breathe, and theircomposition and physiology continue to change after harvest. Thecellular breakdown and death of fresh fruits and vegetables areinevitable but may be slowed with optimal storage conditions. Water lossmay result in weight loss, wilting, and shriveling. The loss of water isdriven by the VPD at the surface of the food. The greater the VPD at thesurface of the food, the greater and more rapid the loss of water fromthe food product. This loss of moisture can lead to the decay of thestored food product. As discussed above, in most circumstances the VPDat the surface of the food is closely associated with the VPD ascalculated form the surrounding air and the two may be usedinterchangeably.

On the other hand, if the VPD is low and the temperature decreased, theamount of water in the air can exceed the saturation vapor pressure.When this occurs, the water will condense out of the air and coalesce onthe surfaces of the refrigerator and the food products stored therein.This free, liquid water on the fresh fruits and vegetables can also leadto decay of the stored food product due to pathogen growth.

Another important consideration in refrigerated food storage istemperature control. Freezing fresh fruits and vegetables can lead todamage due to ice crystal formation and the resultant damage to thecellular structure of the fruits and vegetables. Conversely, elevatedstorage temperatures can allow for accelerated growth of pathogens,which leads to decay. Accordingly, in an example embodiment of thepresent subject matter, in addition to the tight control of the VPD,control of the temperature of the food storage compartment within arange safe for food storage is required. In an example embodiment, thetemperature range that is safe for food storage is from 32° F. to 45°F., from 33° F. to 42° F., or from 34° F. to 37° F.

As can be appreciated from the equations above, VPD is dependent ontemperature and humidity. As previously noted, in a refrigerator, it iscommon to have direct control the temperature of a compartment. Thus, asrepresented in FIG. 3 , temperature is typically maintained a within atight range (e.g., +1° C.). However, this produces large fluctuations inVPD because a typical refrigerator is not equipped with an activehumidifier. As represented in FIG. 4 , in an example embodiment of thepresent subject matter, tight control of the VPD is achieved andmaintained by loosening the control of the temperature and allowing thetemperature to float within temperature range that is safe for foodstorage. As used herein, “tight control” means that the VPD ismaintained within +10% of the desired range. However, the temperature isonly allowed to float within a temperature range that is safe for foodstorage, e.g., 32° F. to 45° F. If the temperature within the foodstorage compartment reaches the limit for safe food storage, then thesealed system will be activated or deactivated, as required, to returnthe temperature within the food storage compartment to a safetemperature for food storage, regardless of the VPD.

Schematically, if the determined VPD is higher than the desired range,then the temperature is lowered (by engaging the cooling cycle) to lowerthe saturation vapor pressure. Theoretically, if the absolute humidityinside a compartment is kept constant, in other words, if there is nomoisture generation or loss inside the compartment, the VPD decreases ifthe temperature is allowed to decrease. This is because the saturationpressure of water vapor decreases rapidly with temperature whereas theactual vapor pressure of water inside the compartment decreases soslightly that it can be essentially considered a constant. Conversely,if the determined VPD is lower than the desired range, then thetemperature is allowed to increase (by disengaging the cooling cycle) toincrease the saturation vapor pressure. Again, theoretically, the vaporpressure is essentially constant and the increase in the saturationvapor pressure would increase in VPD.

However, under real world conditions, the operation of the sealed systemto regulate temperature involves the operation of an evaporator.Accordingly, during cooling, humidity is initially removed to cool theair in the compartment. Thus, depending on humidity, fan speeds, andstarting temperature, the operation of the sealed system can lowertemperature while actually increasing VPD due the removal of moisture byevaporation. This can be mitigated in various ways. For example, thefans can be operated at higher speeds or a fan defrost of the evaporatorcan be carried out to re-introduce the moisture into the compartment. Insum, the initial operation of the sealed system may increase VPD, butonce the compartment allowed to equilibrate, the VPD will decrease asdesired.

In an example embodiment of the present subject matter, the refrigeratormay include a mechanism for heating the food storage compartment.Examples mechanisms for such heating include a defrost heater, a meatpan heater and the like. In an example embodiment, the heater may beused in conjunction with the sealed system to provide control over thetemperature in the food storage compartment, and as discussed above,provide tight VPD control.

In an example embodiment of the present subject matter, the desired VPDrange is a VPD range that produces relatively small loss of water fromthe stored food product, while also avoiding condensation in the foodstorage compartment. Generally, the desired VPD range will be dependenton the type of food being stored and will avoid approaching a VPD of 0to prevent or reduce condensation. In an example embodiment of thepresent subject matter, the desired VPD range is between about 0.05 andabout 0.85, between about 0.05 and about 0.50, between about 0.07 andabout 0.15, between about 0.15 kPa and about 0.22 kPa, between about0.22 kPa and about 0.30 kPa, between about 0.30 kPa and about 0.37 kPa,or between about 0.30 kPa and about 0.45 kPa.

In an example embodiment of the present subject matter, the desiredrange for the VPD may be set using any convenient mechanism. Themechanism for setting the desired range for VPD may be entry of thedesired range into the controller during manufacture, i.e., a defaultsetting. The mechanism for setting the desired range for VPD may be theinput 220 incorporated into the refrigerator 100 or connected remotelyto the controller allowing the user to select the desired range. In anexample embodiment, the input 220 may be a touch pad, a button orplurality of buttons, a dial, or any other user interface, adapted toallow the user to manually input the desired range for VPD. In anotherexample embodiment, the input 220 allows the user to input a numericrange for the VPD. In yet another example embodiment, the input 220allows the user to select a food to be stored and the controller selectsan appropriate desired range for VPD based on the food selected.

In example embodiments of the present subject matter, the refrigerator100 may include a plurality of food storage compartments 110 and mayinclude VPD control in any one or more of the plurality of food storagecompartments. In such an example embodiment, each food storagecompartment may include a separate thermometer and hygrometer. Moreover,each food storage compartment may individually cooled or not, asappropriate by the controller and sealed system. In an exampleembodiment, one or more of the food storage compartments may kept undertight VPD control and one or more other food storage compartments may bekept under conventional tight temperature control, as desired by theuser.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

What is claimed is:
 1. A method of preserving food inside arefrigerator, the refrigerator comprising a compartment for storingfood, the method comprising: measuring the humidity in the compartment;measuring the temperature in the compartment; determining the vaporpressure deficit in the compartment; and adjusting the temperature inthe compartment to maintain the vapor pressure deficit in thecompartment within a desired range.
 2. The method of claim 1, whereinthe desired range for the vapor pressure deficit is between about 0.07kPa and about 0.15 kPa.
 3. The method of claim 1, wherein the desiredrange for the vapor pressure deficit is between about 0.15 kPa and about0.22 kPa.
 4. The method of claim 1, wherein the desired range for thevapor pressure deficit is between about 0.22 kPa and about 0.30 kPa. 5.The method of claim 1, wherein the desired range for the vapor pressuredeficit is between about 0.30 kPa and about 0.45 kPa.
 6. The method ofclaim 1, wherein the temperature in the compartment is adjusted tobetween 33° F. and 45° F.
 7. The method of claim 1, wherein thetemperature in the compartment is adjusted to between 34° F. and 37° F.8. The method of claim 1, further comprising: detecting the food in thefood storage compartment and selecting the desired VPD range based onthe food detected.
 9. The method of claim 8, wherein the refrigerator isoperated in a manual VPD control mode, a semiautomatic VPD control mode,or an automatic VPD control mode.
 10. A method of preserving food insidea refrigerator, the refrigerator comprising a plurality of compartmentsfor storing food, the method comprising: measuring the humidity in eachof the plurality of compartments; measuring the temperature in each ofthe plurality of compartments; determining the vapor pressure deficit ineach of the plurality of compartments; and selecting vapor pressuredeficit control for at least one of the plurality of compartments,wherein a) for each of the plurality of compartments for which vaporpressure deficit control is selected, individually adjusting thetemperature in each of the selected compartments to maintain theindividual vapor pressure deficit in each of the selected compartmentswithin an individually desired range; and b) for each of the pluralityof compartments for which vapor pressure deficit control not selected,individually adjusting the temperature in each of the unselectedcompartments within an individually desired range without regard to thevapor pressure deficit in any of the unselected compartments.
 11. Themethod of claim 10, wherein the desired range for the vapor pressuredeficit in at least one of the selected compartments is between about0.07 kPa and about 0.15 kPa.
 12. The method of claim 10, wherein thedesired range for the vapor pressure deficit in at least one of theselected compartments is between about 0.15 kPa and about 0.22 kPa. 13.The method of claim 10, wherein the desired range for the vapor pressuredeficit in at least one of the selected compartments is between about0.22 kPa and about 0.30 kPa.
 14. The method of claim 10, wherein thedesired range for the vapor pressure deficit in at least one of theselected compartments is between about 0.30 kPa and about 0.45 kPa. 15.The method of claim 10, further comprising: detecting the food in atleast one of the selected compartments and selecting the desired VPDrange at least one of the selected compartments based on the fooddetected.
 16. The method of claim 15, wherein the refrigerator isoperated in at least one of the selected compartments in a manual VPDcontrol mode, a semiautomatic VPD control mode, or an automatic VPDcontrol mode.
 17. A refrigerator comprising: at least one food storagecompartment; at least one thermometer; at least one hygrometer; a sealedsystem configured for cooling air in the at least one food storagecompartment; and a controller configured to: 1) receive a measuredtemperature from the thermometer and a measured humidity from thehygrometer and to calculate a vapor pressure deficit based on themeasured temperature and measured humidity, 2) activate or deactivatethe sealed system, wherein: when the vapor pressure deficit is within adesired range, the controller deactivates the sealed system; or when thevapor pressure deficit is outside of the desired range, the controlleractivates the sealed system.
 18. The refrigerator of claim 17, whereinthe refrigerator is operated in a manual VPD control mode or asemiautomatic VPD control mode.
 19. The refrigerator of claim 17,further comprising a food detection device, and wherein the fooddetection device detects the food in the food storage compartment andthe controller selects the desired VPD range.
 20. The refrigerator ofclaim 19, wherein the food detection device is a camera.